Whoever thought the referendum would be this close? Die-hard Brexiteers may claim they knew it all along but many investors are only just waking up to the fact that Britain really could vote to quit the EU on 23 June as the campaign takes a 7% lead according to some polls.

Remain holds the day

Momentum is with Leave as The Sun newspaper hops on-board and exhausted pro-Remain campaigners complain that most traditional Labour voters want out, and the FTSE 100 (INDEXFTSE: UKX) doesn’t like it. The index lost £100bn in four straight days and broke through the 6,000 barrier on Tuesday after falling 2.01%. It recovered slightly to end Wednesday on 5,966.80.

Of course, it hasn’t really “lost” £100bn, the misplaced money will be “found” soon enough if Bremain holds sway and the index enjoys the mother of all relief rallies.

The tighter the vote looks set to become, the more volatile share prices will be in the week ahead. The same goes for sterling, with currency markets only just waking up to how tight the vote is. Wealthy London-centric financial service professionals have been slow to recognise rumbling anti-EU anger in the forgotten provinces.

I’m a be-Leaver

I like to underpin my portfolio with passive UK funds that track either the FTSE 100, FTSE 250 or the All-Share, and I’m always looking for a chance to top them up when the market dips. One month ago the FTSE 100 closed at 6,353. At today’s starting price I would pick up 7% more stock for the same money. The index is almost 17% lower than in April 2015, when it peaked at 7,122. So the Brexit buying opportunity is already upon us.

If Brexit sweeps to victory, the FTSE will plunge and could easily breach 5,000. Now that really would be a once-in-a-lifetime opportunity, because once we vote out we’re out for good. There’s no way back in. The shock will be seismic and shares will be far cheaper than they are today.

Will we stay or will we go?

Nobody knows how the UK will vote next week (if you DO know, get thee to Paddy Power). If Remain holds the field, the FTSE 100 will soar and you’ll be delighted you bought in the days before the referendum. If Leave is triumphant, you could take an instant hit, but at least have an opportunity to buy more stock at the new (much) lower price.

When to buy is a decision that only you can make. Personally, I’m looking to split my investment vote and take advantage of pre-referendum volatility to top up my FTSE All-Share tracker at today’s reduced price. That way I can feel smug if we vote to Remain and the index rebounds. 

If we vote to Leave, I’ll be bracing myself to swim against the tide of panic sellers, and buy more. I intend to keep my money invested for at least 20 years, and over such a lengthy timeframe can afford to look beyond political earthquakes like the one heading towards us.

Brexit is a great buying opportunity. Don’t waste it.

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Harvey Jones has no position in any shares mentioned. The Motley Fool UK has no position in any of the shares mentioned. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.